Multigeared bicycles typically are shifted manually under the control of the cyclist. Generally, multigeared bicycles have a gear assembly at the rear wheel (rear gear assembly), and one at the pedal-crank assembly (front gear assembly). The cyclist can independently select a front gear and a rear gear. By selecting appropriate front and rear gears, a cyclist determines a gear ratio that is suitable for the riding conditions. For example, a cyclist may select a low gear ratio when traveling uphill and a high gear ratio when traveling downhill at a high rate of speed. The gear ratio specifies the number of revolutions that the rear wheel turns per revolution of the pedal crank assembly. Each combination of a front and rear gear is referred to as a bicycle gear. The bicycle gears are ordered from gear 1 (low gear) based on the gear ratio. In the following, the term "bicycle gear" refers to a particular front and rear gear combination.
Many cyclists find it desirable and comfortable to travel at a constant cadence. The cadence is the speed at which the cyclist pedals. To maintain a constant cadence, the cyclist shifts gears based on the riding conditions. For example, a cyclist may select a desired cadence. The cyclist may be comfortable pedalling at this cadence when the bicycle is in the highest gear traveling on level terrain. If, however, the cyclist encounters a slight rise in the terrain, it may be uncomfortable (too hard) to pedal at the desired cadence. The cyclist shifts to a lower gear so that the desired cadence can be maintained. It is, however, often difficult for a cyclist to maintain a constant cadence. The cyclist may not know which gear should be selected to maintain the desired cadence. Also, the cyclist may have to shift both the front and rear gears to maintain the desired cadence. This can be cumbersome, especially when the terrain changes often.